Chop up the chocolate into small pieces.
Add cream and chocolate to a small saucepan and put on a low heat.
Sift in cocoa powder and add golden syrup.
Mix very well to incorporate all the cocoa powder and melted chocolate. If you are having trouble getting out all the cocoa lumps you can blitz it briefly and very carefully with an immersion blender.
Add brown sugar, vanilla essence and butter.
Cook on low, stirring frequently untill sauce starts to thicken and is smooth and glossy.
To test if the sauce is ready put a small spoonful into a small bowl and put in the freezer for a couple of minutes. If the sauce is nice and thick its done, other wise continue to cook for a bit longer.
Remove from heat and allow to cool slightly. Transfer to an heatproof container and either use immediatly or store in an airtight container in the fridge untill needed.